The method relates to improvements in welded joints in equipment in which a surface layer of a special purpose metal or alloy, such as a metal with excellent corrosion resistance, is used in conjunction with a backing of a more common base metal. The method is versatile, and is adaptable to wall constructions wherein the special purpose metal either is or is not bonded to the backing. The composite wall construction thus has the structural strength of the backing material and the corrosion resistance or other useful property of the special metal at the clad or lined surface.
The improved method of fabrication described herein can be applied to an extremely large number of dissimilar metal combinations. Some of such combinations include: tantalum, niobium (columbium), vanadium, titanium, zirconium, or hafnium, or their alloys, as liners or layers on mild steel; tantalum or niobium layers on copper-base alloys; tantalum plus copper layers on mild steel; and tantalum layer on nickel or nickel-base alloys, or on cobalt-base alloys. The liner or layers of the special purpose metal or alloy may be bonded to the backing by techniques including roll bonding, explosive cladding (such as the Detaclad process, trademark of E. I. duPont de Nemours and Co., described in U.S. Pat. Nos. 3,137,937 and 3,233,312); and brazing, such as with silver brazing alloys; or the lining or facing may simply be in close contact with the backing, such as an unbonded, loose, or thermal-expansion compensated lining.
The use of a special purpose metal or alloy lining or facing on a more common base metal backing is well known in the state-of-the-art. However, methods for fabrication of such equipment present serious problems, especially at joints when the lining material is not generally "metallurgically compatible" with the backing material. An example of such metallurgical incompatibility is the case of utilizing a tantalum liner or layer on a mild steel backing. The melting points of these materials differ widely; tantalum melts at about 3000.degree. C. (5430.degree. F.) and mild steel at about 1530.degree. C. (2790.degree. F.). Mild steel has a coefficient of thermal expansion about twice that of tantalum. Iron and tantalum do not alloy to produce ductile alloys (the tantalum-iron phase system contains the brittle intermetallic compound TaFe.sub.2 and eutectics between this compound and the very limited solid solubility, terminal solid solutions - See FIG. 11, p. 461, "Columbium and Tantalum", by F. T. Sisco and E. Epremian, published by John Wiley and Sons, Inc., (1963).
To overcome these metallurgical incompatibilities, U.S. Pat. No. 3,443,306 to Meyer, utilizes a copper interlayer (about 0.060-inch thickness) between the tantalum cladding and the steel base to serve as a heat dissipating layer during the welding procedures. In this patented method of weld assembly, the thickness of the copper layer is critical, and the copper intermediate layer must be integrally bonded to both the steel substrate and the outer tantalum layer. The final weld joint contains an outer projecting batten strap of tantalum which is joined by fillet welds to the underlying tantalum cladding. The fillet welds at the ends of the projecting batten strap cannot meaningfully be x-ray inspected for code weld quality, so techniques such as soap tests, helium leak tests with a mass spectrometer, or penetrant dye tests are used to monitor weld quality. While such tests can indicate that the initial weld is leak-tight, they do not provide assurance that an adequate depth of weld and weld penetration has been achieved to provide the necessary strength in the joint. Furthermore, the method of joint assembly does not provide provisions for accommodating the internal stresses caused by the differential thermal expansion between the steel and tantalum when the fabricated unit is heated to or cooled from the subsequent service conditions. Thus, such joints can be prone to failure during service from the stresses due to thermal cycling, as well as to fatigue cycling during service.
In joining sections of such composite clad materials, it is necessary to separately preserve the chemical purity and maintain the properties of the clad metal and of the backing metal in their respective weld joints. It is further desired that the welds in the clad metal and backing metal have complete penetration through their respective cross-sections. Additionally, the initial full cross-section thickness should be preserved in the final joints; i.e., the wall of the backing material (say, mild steel) should not be notched or grooved in the joint area. Furthermore, it is desired that the weld joints be capable of permitting inspection of weld quality, such as by x-ray radiography, as they are produced. Finally, it is desired that provisions be provided in the weld joint to compensate for the differences in thermal expansion between the base metal and clad metal to minimize the possibility of mechanical failure in service caused by thermal or oscillating (fatigue) stresses.
A number of variations in wall and joint constructions for dissimilar metal composites have been presented in the prior art. Some of these include the following United States patents:
2,209,290: Watts PA1 4,032,243: Keifert et al PA1 4,073,427: Keifert et al PA1 4,510,171: Siebert
All of these prior art patents are deficient with respect to at least one of the above-outlined desired characteristics or attributes in a dissimilar metal weld joint construction.
One principal object of the present invention is to provide a versatile method of fusion welded joint fabrication for equipment comprising a combination of a lining or facing of a special purpose metal or alloy, especially the refractory and reactive metals and their alloys, and a ferrous or non-ferrous base metal on which the lining or facing material may either be bonded or unbonded.
Another object is to provide a method of fabrication that will simplify and improve the fitup of both the backing and lining and facing in joining either bonded or unbonded equipment in which the wall is constructed of two or more layers.
An additional object is to provide a method of fabrication to produce sound, full penetration and contamination-free welds independently in the base metal and in the lining or facing, wherein the full cross-sectional thickness of both members are maintained, and, furthermore, to allow thorough, non-destructive testing to ensure high quality in the welds in sequence as they are completed.
A further object is to provide means in the joint fabrication to compensate for differences in thermal expansion of the base metal and lining metal. Thus, the possibilities of failures during subsequent service caused by induced thermal and mechanical stresses are minimized.
These and other objects provided by this improved joint construction and method of fabrication will be apparent to those skilled in the art by the following drawings and examples.